Type C viruses have been causally linked to leukemia and sarcoma in a variety of vertebrate species including at least one primate. However, the role of these viruses in human malignancy remains unclear. We have conducted experimentation in several areas to further elucidate this problem. A wide range of human tumor specimens have been analyzed by nucleic acid hybridization for the presence of gene sequences related to the endogenous type C viruses of baboons, macaques, and owl monkeys. There is, at present, no firm evidence that any of these viruses have been horizontally transmitted to man, thereby resulting in malignancy. Moreover, attempts have been made to rescue type C viral sequences from primate and human cell lines by infection with heterologous helper viruses. This technique permitted the isolation of a novel class of murine retroviral-associated sequences and hopefully will allow the isolation of the analogous gene sequences from man.